It is known in prior art in connection with generation of a magnetic field in a substantially cylindrical space parallel to the axis of said space, to employ a coil wound on a cylindrical mantle surface concentric with the cylindrical space. Such coils of so-called solenoid shape are also commonly used for control, frequently including so-called focussing, of charged particles, such as electrons for instance. It is also a known fact that the greater the length of the solenoid in relation to its radius, the greater is the axial extension of the central homogeneous magnetic field. In order that an axial field displaying a high degree of homogeneity might be produced in a cylindrical space of given size, it is necessary that the length of the solenoid is considerably in excess of the length of the space in question. In numerous instances this is not practically feasible owing to space considerations.